As wavelength division multiplexing (WDM) network technology matures, ultra fast optical time division multiplexed (OTDM) packet network are attracting attention because they offer more flexible and efficient communication than WDM networks. It should be noted that all-optical signal processing functions necessary for ultra fast OTDM system (≦40 Gbit/s) are also beneficial even in WDM optical communication systems. If simple and reliable all-optical signal processing technologies are available, these may improve the system design of transmission schemes limited by the electronic devices and instruments. Here, the key is the novel kinds of optical devices that make possible some simple but ultra fast signal processing functions, including the generation of stable ultra short light pulses, beyond the speed limit of the present electronics.
In general, information is coded in the electrical using multiple stage of electronic multiplexer (ETDM). Today, it seems commercially practicable to operate with such a scheme at bit-rates up to 20 Gbit/s and 40 Gbit/s in lab version. However, the main problem is the actual and non-degraded conversion of the data from the electrical to the optical domain. For that purpose, broadband intensity modulators often based on LiNbO3 intensity modulator or integrated electro-absorption modulator which require large peak to peak voltage driver have been considered.
An alternative solution is the use of OTDM (RZ format) using passive optical couplers and delay-lines, which has the main advantage of using lower bitrate electronic components. This well-known solution has a potential to enable very high bit-rate at low costs. However, some problems are still not satisfactory solved, namely that extinction ratio between pulses (requiring best quality pulse sources) are sufficient to minimize possible interferometric noise and that temporal interleave between tributaries are good enough.